1. Field of the Invention
The invention relates to an extruder die head, preferably a blown film head for mono and/or coextrusion. The extrusion die head comprises at least one internal cylindrical mandrel and a shell, which encloses the mandrel concentrically. Between the mandrel and shell an annular channel is formed that empties into a die slit. The extruder die head also comprises at least one line, which feeds a polymer melt and empties into the annular channel in the area opposite the die slit.
2. Related Art
This type of blown film head is well-known. Usually the mandrel exhibits one or several spiral grooves, whose depth tapers off in the direction of the die slit starting from the feed channel(s), so that the polymer melt overflows more and more the spider legs defining the channels and assumes uniform flow in the axial direction. One particular problem associated with these prior art extruder die heads lies in the fact that the mandrel opposite the start of the annular channel is fitted sealingly into a cylindrical borehole of the shell. If, however, the extruder die head heats up due to the polymer melts flowing through the same, the shell expands more, due to its larger diameter, than the central mandrel, so that between both mandrel and shell a slit is formed, into which flows the polymer melt, which is fed in under pressure. Since the melt collects in the region of this slit and decomposes in the hot die head due to its long residence time, particles of the deposited melt can be dragged along by the melt. These particles appear again as defective spots in the extruded film blown tube or the inflated tubular bubble.
Therefore, the object of the invention is to provide an extruder die head of the kind described above, in which no slit can form, during the heating process, between the central mandrel and the shell, which encloses it and in which the polymer melt can accumulated and be deposit and become brittle, are avoided.
The invention solves this problem in that the area of the start of the annular channel between the mandrel and the shell exhibits a ring with such a coefficient of thermal expansion xcex3 that during the heating process it seals the slit between the shell and the ring.
The ring of the invention exhibits a different coefficient of thermal expansion xcex3 than the material of the mandrel and the shell so that due to the greater thermal expansion of the shell the ring seals the slit between mandrel and shell just like a press fit packing.
Preferably a ring, whose coefficient of thermal expansion is greater than that of the shell, is mounted on the mandrel.
Another embodiment provides that a ring, which exhibits a smaller coefficient of thermal expansion than the shell, is mounted on the inside wall of the shell.
A preferred embodiment provides that the rings are held in annular shoulders of the mandrel and/or the shell. In order to slide the rings on, the shell can be provided with a borehole of larger diameter and/or the mandrel with a smaller diameter up to the step shaped shoulder. The rings rests, on the one hand, with one face against the face of the annular shoulder and, on the other hand, with its outer or inner circumference against the shell or the mandrel in the recess.
If the mandrel and the shell are made of steel or a steel alloy, the ring is made preferably of copper, a copper alloy, bronze, brass or a brass alloy.
Another embodiment solves the problem in that the mandrel and the shell, enclosing it, exhibit in such a manner different coefficients of thermal expansion that the seat, which connects the mandrel to the shell and is located below the annular channel, becomes more impervious with heat. In this embodiment the force fit between the mandrel and shell is increased so that any slits that might still be present in the seat area are closed.
Another embodiment solves the problem in that a pipe piece having such coefficients of thermal expansion that the seat, which connects the mandrel to the shell and is located below the annular channel, becomes more impervious with heat, is shrunk into the shell.
Another embodiment solves the problem in that a pipe piece having such coefficients of thermal expansion that the seat, which connects the mandrel to the shell and is located below the annular channel, becomes more impervious with heat, is shrunk on the mandrel. In this case, too, the press fit becomes more impervious so that slits are prevented.
One or several helical groove(s) having a depth that tapers in the direction of the die slit can be milled into the pipe piece or through the pipe piece into the mandrel.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.